1. Field of the Invention
The present invention relates to a variable transmission power type transmitter comprising a transmission interruption detecting circuit.
2. Description of the Related Art
In a mobile communication system such as an automobile telephone system, a portable telephone system, and the like, in order to prevent a mobile terminal from transmitting needlessly, and to reduce power consumption of the mobile terminal, the transmission power of the mobile terminal is controlled to be kept as low as possible while maintaining communication when electrical field strength is sufficiently high at the mobile terminal's position, and the transmission power is turned off when transmission is not required.
In order to realize these functions, a radio transmitter for a base station and a mobile terminal are provided with an automatic power control circuit (APC circuit) which is capable of variable control of the transmission power. The radio transmitter is also provided with a transmission interruption detecting circuit (or CAR-IND circuit: carrier indicator) for detecting existence or absence of the transmission output, in cooperation with the automatic power control circuit. The radio transmitter is further provided with a set point providing circuit for providing the automatic power control circuit with a set point by which the transmission power of the automatic power control circuit is controlled. The set point providing circuit discretely alters the value of the set point between the values corresponding to a minimum power P.sub.min and maximum power P.sub.max, according to a command issued from a processor to control the whole apparatus.
The transmission interruption detecting circuit compares a detected value of a level of the transmitting power with a predetermined reference value V.sub.ref to decide whether the transmission power exists or not. The reference value V.sub.ref is manually adjustable to compensate for dispersion of each element. The values corresponding to the P.sub.min and P.sub.max are also manually adjustable for compensation.
In the aforementioned conventional circuit, adjustment of the reference value V.sub.ref is required after adjustment of the values corresponding to the P.sub.max and the P.sub.min. Accordingly, there are so many portions to be adjusted that several production processes are required. Additionally, the scale of the circuit is large and its price is high, since a variable resistor for adjustment of the reference value V.sub.ref is required.